1. Field of the Invention
The present invention relates to the field of the prevention of the explosion of an element of an electrical transformer cooled by a volume of fluid, particularly of combustible fluid.
2. Description of the Relevant Art
Electric power transformers undergo losses, both in the windings and in the core, requiring the dissipation of the heat produced. High power transformers are therefore generally cooled by a fluid such as oil. The oils used are dielectric and are liable to catch fire above a temperature of about 140° C. Since transformers are very costly elements, their protection demands close attention.
An insulation defect initially generates a strong electric arc which activates the electrical protection systems which trip the power supply cell of the transformer (circuit-breaker). The electric arc also causes substantial diffusion of the energy which causes the liberation of gas, particularly of hydrogen and acetylene, by decomposition of the dielectric oil.
Following the liberation of gas, the pressure in the transformer tank increases very rapidly, causing an often very violent explosion. The explosion causes a considerable rupture of the mechanical links of the transformer tank (bolt, weld), placing the gases in contact with the oxygen of the ambient air. Since acetylene is autoinflammable in the presence of oxygen, a fire breaks out immediately and propagates to the other units on the site, which are also liable to contain large quantities of combustible products.
The explosions are due to insulation failures resulting from short-circuits caused by overloads, surge voltages, progressive damage to the insulation, a low oil level, the appearance of water or mold, or the failure of an insulation component.
The prior art describes fire extinguishing systems for electrical transformers which are activated by fire detectors. These systems are activated when the transformer oil is already burning. It was therefore considered sufficient to limit the fire to the equipment concerned and prevent it from propagating to the neighboring installations.
To slow down the decomposition of the dielectric fluid due to an electric arc, silicone oils can be used instead of conventional mineral oils. However, the explosion of the transformer tank due to the increase of the internal pressure is only delayed for an extremely short period, about a few milliseconds. The explosion of the tank is hence unavoidable.
WO-A 97/12 379 teaches a method for preventing explosion and fire in an electrical transformer equipped with a tank filled with a combustible cooling fluid, by detecting a rupture of the electrical insulation of the transformer by a pressure sensor, depressurization of the cooling fluid present in the tank, using a valve, and cooling of the hot parts of the cooling fluid by injecting a pressurized inert gas into the tank bottom to stir said fluid and prevent oxygen from entering the transformer tank. This method is satisfactory and serves to avoid the explosion of the transformer tank.
WO-A 00/57 438 describes a quick opening rupture element for an explosion prevention device of an electrical transformer.
Unpublished U.S. patent application Ser. No. 11/473,339 to Philippe Magnier describes a preventive device permitting extremely rapid decompression and collection of the fluid passing through the pressure release element in a hermitically sealed reservoir. This reservoir may be equipped with an outlet line which can be connected to a gas pump and an auxiliary reservoir.
The applicant has found that this type of preventive device had drawbacks for transformers placed in confined areas, and for low power transformers for which the cost of the preventive device must be reduced.